Semiconductor Device Package and Method of Assembly Thereof

ABSTRACT

A semiconductor die package includes: an assembly including a semiconductor die, a clip structure attached to an upper surface of the semiconductor die, and a heat sink attached to an upper surface of the clip structure; and a molding material partially encapsulating the assembly, wherein an upper surface of the heat sink is exposed through the molding material.

FIELD OF THE INVENTION

The present invention relates generally to electronic devices and morespecifically to semiconductor packages and method of assembly.

BACKGROUND OF THE INVENTION

It is conventional in the electronic industry to encapsulate one or moresemiconductor devices in a semiconductor package. These plastic packagesprotect a chip from environmental and handling hazards and provide avehicle for electrical and mechanical attachment of the chip to anintended device. The packaging of semiconductor components such as powersemiconductor devices involves a number of design challenges, such ascost, heat dissipation, device protection, size, performance, andreliability among others.

Various approaches to packaging semiconductor devices have beendocumented in the literature as well as commercialized. Some approachesuse lead frames that are stamped into the desired lead configuration onwhich semiconductor devices are attached and wire bonded prior toencapsulation followed by post-encapsulation lead forming, i.e., leadbending and shaping to the desired configuration. This packagingtechnique requires custom trimming and forming machinery and tools.These trimming and forming steps and requisite machinery along with thesolder or epoxy die attachment and wire bonding, ribbon bonding or clipbonding add to production time, complexity and cost.

Many semiconductor die packages use clips instead of wires to formexternal connections to external terminals. Such semiconductor diepackages are sometimes referred to as “wireless” packages. A typicalwireless package includes a clip that is attached to a semiconductordie. Wireless packages generally have better electrical and thermalperformance than packages that use wire-based electrical connections.One such approach is the so-called quad flat pack no-lead (QFN) package.

As noted above, one principal consideration in the package design isheat dissipation. With respect to QFN packages, by way of example,conventional QFN packages dissipate heat from the exposed surface of thelead frame at the bottom of the package. In addition to dissipating heatfrom the bottom surface of the package, it can also be desirable todissipate heat from the upper surface of a device package to pull heataway from a circuit board on which the device is mounted. Although dualheat dissipation (i.e., from both upper and lower surface of a packageddevice) can provide some dissipation advantages, manufacturing andperformance challenges exist that prevent these packages from beingoptimally reliable and cost effective. Such challenges include exposedsemiconductor die surfaces, inconsistent alignment of components, andnon-standardized designs that require expensive tool modifications, toname a few.

Accordingly, new package structures and methods of assembling the sameare desired

SUMMARY OF THE INVENTION

A semiconductor die package includes an assembly including asemiconductor die, a clip structure attached to an upper surface of thesemiconductor die, and a heat sink attached to an upper surface of theclip structure. A molding material partially encapsulates the assembly,wherein an upper surface of the heat sink is exposed through the moldingmaterial.

The above and other features of the present invention will be betterunderstood from the following detailed description of the preferredembodiments of the invention that is provided in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate preferred embodiments of theinvention, as well as other information pertinent to the disclosure, inwhich:

FIG. 1A is a top perspective view of a packaged semiconductor devicehaving an exposed heat sink according to an embodiment of the presentinvention;

FIG. 1B is a bottom perspective view of the packaged semiconductordevice of FIG. 1;

FIG. 2 is a top perspective view of the packaged semiconductor device ofclaim 1 with the molding material hidden to show the packaged componentsthereof;

FIG. 3 is a cross-sectional view of the packaged semiconductor device ofFIGS. 1A and 1B;

FIG. 4 illustrates one method of forming a packaged semiconductordevice;

FIG. 5 illustrates an alternative method of forming a packagedsemiconductor device;

FIG. 6 is a cross-sectional view of another embodiment of a packagedsemiconductor device;

FIG. 7 is a top plan view of an alternative embodiment of a clipstructure for use in a packaged semiconductor device;

FIG. 8 is a cross-sectional view of a mold structure for formingpackaged semiconductor devices;

FIG. 9 is a top plan view of an array of heat sinks for use in a moldstructure for forming packaged semiconductor devices;

FIG. 10 is a top plan view of a mold structure after molding but beforesingulation; and

FIG. 11 is a top plan view of an exemplary embodiment of a heat sink.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description. In the description, relativeterms such as “lower,” “upper,” “horizontal,” “vertical,” “above,”“below,” “up,” “down,” “top” and “bottom” as well as derivative thereof(e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should beconstrued to refer to the orientation as then described or as shown inthe drawing under discussion. These relative terms are for convenienceof description and do not require that the apparatus be constructed oroperated in a particular orientation. Terms concerning attachments,coupling and the like, such as “connected” and “interconnected,” referto a relationship wherein structures are secured or attached to oneanother either directly or indirectly through intervening structures, aswell as both movable or rigid attachments or relationships, unlessexpressly described otherwise.

A semiconductor device package is described herein having improved heatdissipation from a top surface thereof. The device package isparticularly useful for packaging power MOSFET devices, such as powerswitches, which have large current capabilities (e.g., 25-30 A) and thusgenerate significant heat. Dissipating at least some of this heat from atop surface of the packaged device prevents at least some of the heatfrom being dissipated into the substrate (e.g., printed circuit board(PCB)) on which the device is mounted. Preventing heat dissipation intoa PCB substrate is of particular desirability when several devices aremounted on a common board.

FIG. 1A is a top perspective view of a packaged semiconductor device 100according to an embodiment of the invention. FIG. 1B is a bottomperspective view of the packaged semiconductor device. In embodiments,the device packaging is a QFN package having a standard So-8 footprint.However, those familiar with package designs will understand that theheat dissipation techniques described herein may be applied to otherpackage techniques such as wireless So-8 leaded, wireless TO-220, orothers. As can be seen in the figures, the packaged semiconductor device100 has a heat sink 120 (also sometime referred to herein as a “heatslug”) at its top surface that is exposed through the molding material110. As shown in FIG. 1B, in this embodiment, the lead frame 180 is alsoexposed through the molding material 120 at the bottom of the device100, providing a second major heat dissipation surface.

The molding material 110 may be an epoxy molding material or any othersuitable commercially available molding material. In one exemplaryembodiment, the molding material is a CEL series epoxy molding compoundavailable from HITACHI Chemical Co.

FIG. 2 is a top perspective view of an assembly of components inside theprotective molding material 110 of the packaged semiconductor device ofFIGS. 1A and 1B.

FIG. 3 is a cross-sectional view of the packaged device of FIGS. 1A and1B showing the assembly of FIG. 2. As can be seen from the figures, theassembly includes, from top to bottom, heat sink 120, adhesive layer130, clip structure 140, conductive adhesive layer 150, semiconductordie 160, conductive adhesive layer 170 and lead frame 180. In moredetail, the bottom surface of the heat sink 120 is attached to the topsurface of the clip structure 140 by adhesive layer 130. Adhesive layer130 can be a conductive or nonconductive adhesive layer. In embodiments,the adhesive layer is a non-conductive epoxy material. More preferably,the adhesive layer is the same conductive solder layer used forconductive adhesive layers 150, 170, such as a lead-tin solder. As thosein the art will recognize, the bottom surface of the clip structure 140is attached to the top surface of the semiconductor die 160 by theconductive adhesive layer 150. The bottom surface of the die 160 isattached to the top surface of the lead frame 180 by conductive adhesivelayer 170.

The heat sink 120 and the clip structure are preferably formed fromcopper. The exposed surface of the heat sink 120 provides a highlythermally conductive surface to which an external heat sink can beattached in a product. The lead frame is preferably formed fromselectively Ag plated copper.

In exemplary embodiments, the semiconductor die 160 is a power MOSFETtransistor such as a vertical MOSFET. The vertical MOSFET includes asource region (not labeled) and a gate region 164 at a top surface 162of the die 160 and a drain region at the (i.e., bottom) surface. Thesource region may have a source metal (e.g., an Aluminum layer, asolderable top metal or solder bumps) and is electrically coupled to asource clip portion 142 of the clip structure 140. The gate region 164of the die 160 is electrically coupled by a gold, aluminum or copperwirebond 190 to a gate lead portion 184 of the lead frame 180.Alternatively, as described in more detail in connection with FIG. 7,the clip structure can include a gate clip portion for coupling gateregion 164 to gate lead portion 184. The source clip portion 142 has adownset portion or arm 143 that electrically couples to the source leadportion 182 of the lead frame 180. The bottom surface of the die 160includes a drain region that is electrically coupled to the lead frameand its drain lead portions 186.

As explained above, in embodiments the bottom of the lead framestructure 180 is exposed through the molding material 110. The exposedbottom surface of the lead frame 180 provides for an additional drainconnection (i.e., in addition to drain leads 186) as well as anadditional cooling path for the semiconductor die package 100.

As can be seen in the assembly view of FIG. 2 and the cross section ofFIG. 3, in embodiments the heat sink 120 includes a shelf, step or ledge122 formed around at least a part of the periphery of the heat sink 120.This shelf is overmolded with the molding material 110, helping tosecure heat sink 120 within the packaged device 100.

In exemplary embodiments, the components illustrated in FIGS. 1-3 havethe following target thicknesses:

Package Component 4 mils die 8 mils die Lead frame 180 0.200 mm 0.200 mmAdhesive layer 170 0.025 mm 0.025 mm Die 160 0.100 mm 0.200 mm Adhesivelayer 150 0.025 mm 0.025 mm Clip structure 140 0.250 mm 0.250 mmAdhesive layer 130 0.025 mm 0.025 mm Heat sink 120: 0.250 mm 0.250 mmTotal Package Thickness 0.875 +/− 0.05 mm 0.975 +/− 0.05 mm

The thickness of the device can be modified by further thinning of theheat sink 120 to a targeted thickness.

FIG. 11 is a top plan view of an exemplary embodiment of the heat sink120. The cross-hatch illustrates regions of reduced thicknesshalf-etched from the upper surface 126, including arms 124 and ledge122. The upper surface 126 has dimensions of about 3.36×2.82 mm. Thearms 124 each have a width of about 0.4 mm, and the ledge has a width ofabout 0.1 mm extending around the periphery of upper surface 126. Thoseskilled in this art will recognize that this heat sink design isparticularized for the standard QFN So-8 package footprint but caneasily be modified to other package formats. The heat sink design canalso be used with dies having varied dimensions within a given packageformat without redesign.

FIG. 4 illustrates one method of forming a packaged semiconductor device100. More specifically, the method illustrated in FIG. 4 uses a highlythermally conductivity epoxy to couple the heat sink 120 to the clipstructure 140. At step (a), an assembly containing the lead frame 180,die 160 and clip structure 140 is provided. Though not shown, theassembly also includes conductive adhesive layers 150 and 170, e.g.,solder layers. As can be seen at (a), the gate lead portion 184 of thelead frame 180 is wirebonded to the gate region 164 of the top surfaceof the die 160. An exemplary process flow for forming the assembly shownat (a) includes: (i) providing lead frame 180; (ii) applying solder tothe top surface of lead frame 180; (iii) attaching the die 160 to thetop surface of the lead frame 180; (iv) applying solder to the topsurface of die 160; (v) attaching clip structure 140 to the top surfaceof die 160; (vi) heating the assembly to reflow the solder; (vii)performing a cleaning process; and (viii) providing wirebond connection190.

At step (b), a highly thermally conductivity epoxy is applied to the topsurface of the clip structure 140.

Next, at step (c), the heat sink 120 is disposed on the top surface ofthe clip structure 140 and the assembly is heated to cure the epoxy,thereby adhering the heat sink 120 to the clip structure 140.

Last, at step (d), the assembly of step (c) is overmolded with themolding material 110 to form packaged semiconductor device 100 having anexposed heat sink 120 at the top surface thereof and an exposed heatsink (in the form of lead frame 180) at the bottom surface thereof. Ifmultiple chips are formed in the same mold, a sawing/cutting or othersingulation process is performed to provide packaged semiconductordevice 100. Those familiar with molding processes will understand thatsubstrates such as tapes can be employed to prevent the overmolding ofsurfaces that are intended to be left exposed, such as the heat sink 120and lead frame 180.

FIG. 5 illustrates an alternative method of forming a packagedsemiconductor device 100. This illustrated method utilizes a conductivesolder layer as the adhesive layer 130 between the clip structure 140and the heat sink 120 rather than a thermally conductive epoxy. At step(a), an assembly is provided again including a stack including the clipstructure 140, die 160 and lead frame 180. Though not shown, theselayers are adhered together by conductive adhesive layers 150 and 170provided between these layers. Unlike the illustration of step (a) fromFIG. 4, this assembly does not include the wirebond between the gatelead portion 184 and the gate region 164 of the semiconductor die 160.An exemplary process flow for forming the assembly shown at (a)includes: (i) providing lead frame 180; (ii) applying solder to the topsurface of lead frame 180; (iii) attaching the die 160 to the topsurface of the lead frame 180; (iv) applying solder to the top surfaceof die 160; and (v) attaching clip structure 140 to the top surface ofdie 160.

At step (b), a conductive solder paste is applied to the top surface ofthe clip structure 140.

At step (c), the heat sink 120 is disposed on the top surface of theclip structure 140. The assembly of step (c) is then heated to reflowall three solder layers (i.e., layer 170 between lead frame 180 and die160, layer 150 between die 160 and clip structure 140, and the solderlayer 130 from step (c) disposed between the clip 140 and the heat sink120). A cleaning process is then performed.

Next, at step (d), the gate lead portion 184 of the lead frame 180 iswirebonded to the gate region 164 of the semiconductor die 160.

Last, at step (e), the assembly of step (d) is overmolded with themolding material 110 to form packaged semiconductor device 100 having anexposed heat sink 120 at the top surface thereof and an exposed heatsink (in the form of lead frame 180) at the bottom surface thereof. Ifmultiple chips are formed in the same mold, a sawing or othersingulation process is performed to provide packaged semiconductordevice 100. Those familiar with molding processes will understand thatsubstrates such as tapes can be employed to prevent the ovem101ding ofsurfaces that are intended to be left exposed, such as the heat sink 120and lead frame 180.

FIGS. 8-10 provide additional details of an exemplary method of formingpackaged semiconductor devices as discussed above in connection withFIGS. 5 and 6. FIG. 8 is a cross-sectional view of a mold structure 200for use in a mold tool for overmolding the assemblies discussed above toform packaged semiconductor devices. FIG. 8 shows a lead framedesignated generally at 202. An array of semiconductor dies 204 aremounted on the lead frame 202. As discussed above, a layer of solder(not shown) is disposed between the dies 204 and the lead frame 202. Aclip structure 206 is disposed over each die 204. A layer of solder (notshown) is disposed between the dies 204 and the clip structures 206. Theclip structures 206 may be provided as individual clips and located overtheir respective die 204 or be provided as an array or matrix ofattached clips (and/or attached clip sections) for later singulation. Ajig structure 208 is provided to support an array or matrix 210 ofconnected heat sinks. Portions of the jig are shown in phantom. The jig208 serves at least two purposes. First, the jig accurately locates thearray 210 of heat sinks with respect to the lead frame 202, and thuswith respect to the dies 204 and clip structures 206. Second, the jighelps support the weight of the array 210, preventing the undesirablespreading of the solder or epoxy layer disposed between the clipstructures 206 and the individual heat sinks 212 within the array 210.

Turning more specifically to the array 210 of heat sinks, a top planview of an example of such an array is shown in FIG. 9. Assuming a moldblock includes sixty-three die sites, the array 210 includes sixty-threeheat sinks 212. These heat sinks are connected to each other in thearray 210 by arms 216 or other connectors. These connections are cutduring the singulation step after molding and may be half-etched tofacilitate cutting.

FIG. 10 is a top plan view of a molding structure 250 after molding butbefore singulation. Molding compound 260 surrounds the exposed heatsinks 212. Dashed border 262 illustrates cut lines for singulating anindividual packaged semiconductor device from the mold structure.

One particular benefit of the heat sink design disclosed herein is thatthe same assembly tooling (e.g., mold and mold tooling) used to form astandard packaged semiconductor device without an embedded heat sink canbe used to form a packaged semiconductor device with the heat sinkdisclosed herein. As the heat sink design does not affect thefunctionality of the package, the designer can switch back and forthbetween using and not using the heat sink, as desired, without having toredesign the package, mold or tooling. The mold/tooling combinations andsettings are essentially the same, only when the heat sink is not used,that portion of the processing specific to the heat sink placement inthe mold is skipped. Further, the same heat sink design can also be usedwith dies of different sizes. This allows for formation of differentdevices using the same tooling and process and even allows for thesimultaneous formation of different devices in the same mold.

FIG. 6 is a cross-sectional view of an embodiment of a packagedsemiconductor device 100A. The assembly of FIG. 6 is identical in allrespects to the device shown in FIG. 3 except for the provision of animprove moisture trap shown generally in area “A”. Having an exposed topside structure, such as heat sink 120, can provide a moisture path fromthe surroundings to the die. The moisture path follows the interfacebetween the internal components (e.g., heat sink 120 and clip structure140) and the molding material to the die 120. By providing a doublemoisture trap around the periphery of the internal components, theingress of moisture is averted or at least delayed. Because the die isspaced from the surrounding environment by not only the clips structure140 but also the heat sink 120, a double moisture trap is created. Inone embodiment, the double moisture trap is bolstered by misaligning theedges of the heat sink 120 and the clip structure 140A, therebyproviding a convoluted moisture path to the semiconductor die surface.Additional moisture protection is provided by periphery shelves 122 and145 formed in the clip 120 and clip structure 140A, respectively.Finally, additional moisture protection is provided by extending theclip structure 140A beyond the lateral edges of the die 160 in theregion A shown in FIG. 100A. All or some of these features can be usedto delay or prevent moisture ingress to the die 120.

As noted above, the heat slug 120 and clip 140A is also designed with,for example, peripheral ledges to help lock the components within theencapsulating packaging material.

FIG. 7 is a top plan view of an alternative embodiment of a clipstructure 140B that may be used in various embodiments of the disclosedpackaged semiconductor device. The clip structure 140B is identical tothe clip structure 140 shown in FIG. 2 only the clip structure includesa gate clip structure 147 having a downwardly depending leg 147 (shownin phantom) for contacting gate lead 184. The gate clip structure 146replaces the wire bond 190, thus simplifying the assembly process. Asdescribed in copending and commonly assigned U.S. patent applicationSer. No. 11/464,333 filed Aug. 14, 2006 and entitled “Wirebond-lessSemiconductor Package”, the entirety of which is hereby incorporated byreference herein, the gate clip structure 146 can be attached to themain body (e.g., source) portion of the lead frame structure 140B andthen cut or otherwise electrically separated from that portion aftermolding, such as in a singulation step.

When assembled, the heat sink 120 does not touch the gate clip 146. Thegate clip 146 is electrically isolated from the source clip structureand the heat sink 120. In some applications, such as when the top sideof the exposed heat sink 120 is to be soldered to an external heat sink,the gate clip needs to be hidden. If the gate clip is exposed at the topsurface in these applications, it could be shorted to the heat sink. Inthe illustrated embodiment, the gate clip 146 is not exposed at theupper surface of the packaged semiconductor device and, therefore, thereis no need to half etch the gate clip to hide it. The gate clip 146,therefore, exhibits improved strength during coining and lead bendingwhen compared to half-etched gate clip structures in the prior art andis more resistant to torsion/tensional stresses during molding.

Although the packaged device is shown with only a single die perpackaged device, it should be understood that the packaged device mayinclude one or more than one semiconductor die, as well as one or morethan one exposed heat sink. It should also be understood that inexemplary embodiments the clip structure is coupled to a lead frame, butin other embodiments a lead frame is not required, such as when apackage is employed and fabricated as disclosed in copending, commonlyassigned U.S. patent application Ser. No. 11/205,687 entitled“Semiconductor Die Package” and filed Aug. 17, 2005, the entirety ofwhich is hereby incorporated by reference herein. In the '687Application, since the lead contacts of the clip-type structure are notcontacted to a lead frame, a tape or other covering means is used tocover the ends of the lead contacts in order to prevent overmoldingthereof.

Further, although the semiconductor die is described as being a powerMOSFET with source and gate terminals on an upper surface thereof anddrain terminal at a bottom surface thereof, it should be understood thatthe die could be configured with drain and gate terminals at an uppersurface thereof and source terminal at a bottom surface thereof.Therefore, a device can be described as having a pair of source/drainterminals and each of these terminals can be referred to as anindividual source/drain terminal from the pair of source/drainterminals. Further, the semiconductor device need not be a verticaldevice. The semiconductor device could be a horizontal device wherecurrent flows horizontally and input(s) and output(s) are at the sameside of the die. Quasi-vertical devices, where current flowshorizontally and vertically may also be utilized.

Although the invention has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the appended claimsshould be construed broadly to include other variants and embodiments ofthe invention that may be made by those skilled in the art withoutdeparting from the scope and range of equivalents of the invention.

1. A method of forming a packaged semiconductor device, comprising thesteps of: forming an assembly comprising a lead frame, a clip structure,a semiconductor die disposed between the lead frame and the clipstructure, and a heat sink attached to an upper surface of the clipstructure; and encapsulating the assembly with a molding material,wherein an upper surface of the heat sink is exposed through the moldingmaterial.
 2. The method of claim 1, wherein the forming step comprisesproviding a plurality of semiconductor dies with corresponding clipstructures and heat sinks, the method further comprising the step ofcutting the encapsulated assembly to provide a plurality of packagedsemiconductor devices.
 3. The method of claim 2, wherein the formingstep comprises providing an array of connected heat sinks, wherein thearray is cut during the cutting step to detach the heat sinks from thearray.
 4. The method of claim 1, wherein the forming step comprises thesteps of adhering the lead frame, die, clip structure and heat sink inthe assembly with respective layers of solder.
 5. The method of claim 1,wherein a lower surface of the lead frame is exposed through the moldingmaterial.
 6. The method of claim 1, wherein the die is a power MOSFETand wherein the clip structure includes a gate clip electricallyconnecting a gate terminal of the power MOSFET to a gate lead portion ofthe lead frame.
 7. The method of claim 1, wherein the encapsulating stepis performed using a mold and a molding tool, the method furthercomprising the step of using the mold and tool to encapsulate a secondassembly to form a second packaged semiconductor device without anembedded heat sink.